(a) Field of the Invention
The invention concerns a tundish for the continuous casting of a steel between ladle and ingot molds.
(b) Description of Prior Art
Tundishes for continuous casting of steel are made of a refractory which has been cast with substantial thickness on a frame of sheet metal defining the outer wall of the tundish and on which has been laid a wear refractory which is restored after each operation.
In order to satisfy increasing requirements on the quality of cast products, steel makers use the tundish as a metallurgical tool which also permits to render the temperature and the composition of the liquid metal uniform and to eliminate all or part of the remaining inclusions. For this purpose, many improvements have enabled to improve the flow of liquid metal, and this has been made possible by modifying the geometry and adding dams (small walls, baffles...). It has also been found that injections of neutral gas (argon, helium, nitrogen...) are complementary to the present means used to improve the metallurgy of the tundish.
If the injection of neutral gas in a liquid metal is now very much in use in ladle metallurgy, the characteristics required from a gas injector in a tundish are substantially different, since in addition to metallurgical requirements, safety requirements and costs limitations are also involved. For these reasons, the techniques used in ladle such as porous plugs and lances cannot be directly transposable. Indeed, on the one hand metallurgical considerations require gas bubbles which are as small as possible through an important part of an injection surface and with a distribution which is as good as possible. In addition, maintenance of this injector should be easy to implement so as not to increase the costs of operation on the tundish. The excess cost involved by this technique depends on the number of tons which are being cast (from 30 t to 1000 t depending on machines used and the lengths of the operations). The tundish is normally placed on a carrier at man's height, on the casting floor, so as to make it possible to have access to the ingot molds. For the safety of the operators, any tapping should therefore be definitely prevented.